Vehicle steering relies upon friction between the vehicle tyres and the ground. Where high levels of friction are available, such as in normal highway driving, understeer and oversteer are seldom encountered. However where friction is reduced, as in mud or snow, the rate of turn of a vehicle may vary substantially from that indicated by steering angle, and in consequence the vehicle may understeer or oversteer.
In such circumstances, the experience of the vehicle driver plays an important part in retaining control of the vehicle and achieving the intended rate of turn.
A vehicle may include a cruise control mode whereby a pre-set speed is maintained regardless of topography. Cruise control systems are widely used in highway driving, generally above about 30 kph, where steering angles are typically small. Autonomous cruise control (ACC) systems permit one vehicle to follow another at a predetermined separation whilst accommodating speed variations of the leading vehicle.
Off-road cruise control has been proposed whereby a vehicle may maintain a pre-set speed over variable terrain. Such a system has the advantage that the vehicle driver, particularly a novice driver, can concentrate upon steering whilst allowing vehicle systems to automatically select a suitable transmission ratio and engine speed. Thus the work rate of the vehicle driver may be substantially reduced. However in the case of high steering angles on low friction surfaces some side slip (understeer) may occur, so that the vehicle does not follow the intended path. An inexperienced driver may not know how to best control the vehicle in such circumstances, in order to make progress whilst avoiding risk or damage.
Mere disengagement of a cruise control mode in difficult conditions places the inexperienced driver under a greater workload, since engine speed and transmission ratio must be controlled in addition to steering.
What is required is a system and method of providing for a prescribed rate of turn of a vehicle in low friction conditions and which is also suitable for use whilst in a cruise control mode.